Electrocoagulation is a simple and effective method of generating metal hydroxide coagulants within water/wastewater by electro-oxidation of metals, such as iron and aluminum. Schematic and basic concept of electrocoagulation is presented in FIG. 1. When current is passed through the system, metal ions are released from sacrificial anode, whereas hydroxide ions and hydrogen gas are produced at cathode. The metal ions and hydroxide ions readily combine to form polymeric metal hydroxides, which are excellent coagulants.
Reactions that occur at the anode, cathode and bulk liquid are shown below with iron and/or aluminum as anode material;
At the anode;
Oxidation of metal;Fe(s)→Fe2++2e−Fe2+→Fe3++e−Oxidation of water;2H2O→O2+4e−+4H+At the cathode;
Production of hydrogen gas and hydroxide ions;2H2O→H2+2OH−4e−+4H+→2H2 In bulk liquid; Formation of metal hydroxides:Fe2++2OH−→Fe(OH)2 Fe3++3OH−→Fe(OH)3 
In the case of an aluminum anode, the corresponding oxidation of the metal will be;Al(s)→Al3++3e−and in bulk liquid the formation of metal hydroxide will be;Al3++3OH−→Al(OH)3 
Depending of the bulk liquid pH, mono- and polyhydroxides of the metals can be formed as follows;Fe(OH)2+,Fe(OH)2+,Fe(OH)2,Fe(OH)3,Fe(OH)63−,Fe(OH)4−,and in case of aluminum used as anode material examples of formed hydroxides are;Al(OH)2+,Al(OH)3,Al(OH)4−,Al13(OH)345+,Al13(OH)327+
The metal hydroxides form flocculants and remove the contaminants by ion exchange, adsorption, neutralization and sweep-floc action. Metal cations neutralize negative surface charges of colloidal particles and pollutants. Then the particles approach closer and agglomerate under the influence of van der Waals forces. Besides various types of ions, organic substances may effectively be removed in the form of sediment flocs by electrocoagulation. Factors such as current density, pH, electrode type, reaction time, etc., play important roles in the formation of metal hydroxides. Generally, higher current density produces more metal ions in the solution. Likewise, solubility of the metal hydroxides largely depends on pH of the solution. Optimum pH range for the formation of iron and aluminum hydroxide flocculants is between 5 and 8. However, the flocs become soluble at lower or higher pH values. pH is equally important for the interaction of metal flocculants with pollutants. For example, removal of fluoride by aluminum hydroxide is thought to be an ion exchange process. Fluoride ion replaces one hydroxide ion from aluminum hydroxide and makes a complex according to the following reaction;Al(OH)3+xF−→Al(OH)(3-x)Fx(s)+OH−
In the patent literature a number of processes and materials related to electrocoagulation for remediating of water or wastewater are disclosed.
US2012/0055871 relates to an electrolysis method that uses iron particles and an aqueous solution containing sodium hypochlorite. The method characterized in that said method uses direct current, the iron particles from the anode and the sodium hypochlorite concentration of the aqueous solution is at least 1 g/L. The document further relates to a method and to a plant for the pretreatment of raw water for producing water that can be easily treated in order to produce drinkable water or so called technical water.
US2005/0167285 provides an electrochemical method for the removal of arsenate from drinking water, wherein the arsenate is removed by adsorption to metal hydroxide, formed by “in-situ” anodic oxidation. The application describes an electrochemical cell fitted with an anode of mild steel or aluminum plate and stainless steel cathode with an inter-electrode distance of 0.5 to 1.5 cm.
EP 0595178A1 describes a device and a process for electrolytic treatment of waste water. The water to be purified and treated first flows through at least one anode chamber containing an insoluble anode and then through another anode chamber containing a soluble permeable anode. Coke particles are added before water enters the soluble anode chamber to initiate a galvanic coagulation process. The porous sacrificial anode can be made of iron filings.
U.S. Pat. No. 4,014,766 discloses a process wherein wastewater is subjected to electrolysis in an electrolytic cell having an anode comprising an insoluble central electrode and a body of particulate iron pieces disposed there around and in electrical contact therewith, whereby impurities in the wastewater become occulted within flocculants of iron hydroxide formed by electrolytic dissolution of the iron pieces, and the flocculants containing the impurities is subjected to oxidation processing and is thereafter separated. A magnetic field can be applied to the wastewater thus treated thereby to promote sedimentation of the flocculants. Reduction of hexavalent chromium ions to trivalent chromium ions can be carried out efficiently in neutral or alkaline condition without using acidic condition which was the case when iron plates were used.
US2006/0000784 describes a method of water or wastewater treatment for removal of pollutants in at least two-step process comprising (a) treatment of water producing at least partially treated intermediate effluent, (b) treatment of the intermediate effluent with a sacrificial metal and producing ions of said sacrificial metal, and providing very thoroughly treated effluent, (c) recuperating sacrificial metal ions generated in step (b) and recycling the recuperated ions in the step (a), the recuperated and recycled ions from the step (c) improve treatment efficiency of step (a) by additionally removing pollutants from the intermediate effluent using recuperated ions, resulting in cleaner intermediate effluent and therefore the pollutant loading in step (b) is reduced, intermediate effluent is further treated more thoroughly, and the demand for sacrificial metal in step (b) is reduced. Step (a) can preferably be biological, biological-abiotic, physical chemical or combinations of these steps.
A variety of pollutants is supposed to be removed such as, heavy metals, organic compounds, dissolved substances, suspended solids, solid particles, nutrients, ammonia, nitrates, nitrites phosphates, microorganisms etc.
WO 2013/059964 A1 describes a self-assembling high surface area electrode which includes an electrode substrate, magnetic electrode particles and a magnetic field source. Under the influence of the magnetic field source, the magnetic particles assemble on the surface of the electrode substrate. The electrode can be used as an anode and/or a cathode in an eltrocoagulation system for treating contaminated feed water. The document further mentions that a suitable material for magnetic electrode particles is iron particles.
EP0015057A2 describes an invention which provides electrodes for water electrolyzers that are cheaper than electrodes composed wholly of sintered particles and gives as good, or lower overpotentials in the release of gases as electrodes composed wholly of sintered materials. The electrodes according to this document have an impervious, electrically conductive substrate, and bonded to the substrate, a porous metallurgically bonded layer from 25-275 μm thick made of nickel, nickel-iron alloy, iron or iron-carbon alloy particles having a diameter of 2 to 30 μm that are sintered to a theoretical density of from 45 to 65%.
Electrocoagulation has been used for a number of years and has been proved to be an efficient method for removal of various pollutants from water and wastewater. However, there is still a need for improving the process, especially related to simultaneous removal of multiple contaminants including fluoride ions and to obtain a process having increased energy efficiency.